Synthesis of permethyldodecaborate and paramagnetic dodecaborate salt

ABSTRACT

The dodecamethyl closo-borane dianion [closo-B 12 (CH 3 ) 12 ] 2−  and anion [closo-B 12 (CH 3 ) 12 ] −  were synthesized and characterized. Dodecamethyl-closo dodecaborate (2−) was produced from [closo-B 12 H 12 ] 2− , using trimethylaluminum, and methyl iodide and modified Friedel-Crafts reaction conditions. The anion was produced from the dianion by chemical oxidation using ceric (4) ammonium nitrate in acetonitrile. The anion and dianion were both characterized by  1 H and  11 B NMR spectroscopy, high-resolution fast atom bombardment (FAB) mass spectrometry, cyclic voltammetry, and single-crystal X-ray diffraction. The “camouflaged” polyhedral borane anion [closo-B 12 (CH 3 ) 12 ] 2− , can be used as a precursor to materials that offer a broad spectrum of novel applications ranging from drug applications and supramolecular chemistry to use as a weakly-coordinating dianion.

The present invention relates to unique three-dimensional methylatedicosahedral boron cage compounds This invention was made with supportunder Contract Number DE-FG02-9ER61975 awarded by the U.S. Department ofEnergy. The Government has certain rights in the invention.

BACKGROUND

Hawthorne et al has reported the preparation of closo-1,12-C₂B₁₀(CH₃)₁₂and other octamethyl C₂ boranes.(Jiang, W., Knobler, C. B., Mortimer, M.D., Hawthorne, M. F., Angew. Chem., 1995, 34, 1332.). The permethylatedicosahedral carboranes [closo-CB₁₁(CH₃)₁₂]³¹ , and the uncharged blueradical closo-CB₁₁(CH₃)₁₂ derived from that anion, has been reported byMichl et al. (King, B. T., Zanousek, B. G., Trammell, M., Noll. B. C.,Michl, J., J Am. Chem. Soc., 1996, 118, 3313) Additionally, otherpersubstituted polyboron compounds are known.

The most prominent persubstituted polyboron compounds is thepolyhydroxylated boron compound, boric acid, B(OH)₃. Alkaline solutionsof B(OH)₃ deposit Na₂[B₄O₅(OH)₄]·nH₂O, which constitute two abundantboron minerals, kernite (n=2) and borax (n=8) (F. A. Cotton, G.Wilkinson, Advanced Inorganic Chemistry, 5th ed., Wiley, New York, 1988,pp. 164-169). Other common boron structures include the trigonal andtetrahedral boron-oxygen units common to borate minerals (G. A. Heller,Top. Curr. Chem. 1986, 131, 39-98) and the icosahedron. The allotropesof elemental boron, (J. Donohue, The Structures of the Elements, Wiley,New York, 1974, pp. 48-82), boron-rich solids (H. Hubert, B. Devouard,L. A. J. Garvie, M. O'Keeffe, P. R. Buseck, W. T. Petuskey, P. F.McMillan, Nature 1998, 391, 376-378) and the parent anion of thepolyhedral boranes, [closo-B₁₂H₁₂]²⁻ (J. A. Wunderlich, W. N. Lipscomb,J Am. Chem. Soc. 1960, 82, 4427-4428) all contain B₁₂ icosohedra.

The charge-delocalized icosohedral ion [closo-B₁₂H₁₂]²⁻ may beconsidered as the parent aromatic species for borane chemistry in amanner similar to that served by the benzene ring in organic (carbon)chemistry. Isoelectronic substitution of one or two :B-H vertices in[closo-B₁₂H₁₂]²⁻ by :C-H⁺ provides the aromatic derivatives[closo-1-CB₁₁H₁₂]⁻, and a set of three isomeric dicarbacarboranes (1,2−or ortho; 1,7− or meta; and 1,12− or para) closo-C₂B₁₀H₁₂ R (N. Grimes,Carboranes, Academic Press, New York, 1970, p. 8). Each of theseisoelectronic derivatives of [closo-B₁₂H₁₂]²⁻, undergoes characteristichydrogen-substitution reactions at their B-H vertices resulting in ahuge number of known icosohedral species (Hawthorne, M. F., “CarboraneChemistry at Work and Play”, Proceedings of the Ninth InternationalMeeting on Boron Chemistry published in Advances in Boron Chemistry,Seibert, W. (Ed.), The Royal Society of Chemistry, London, 1996,261-272)

Of special interest are derivatives in which every available B-H vertexhas been substituted. Thus, hydrophobic derivatives of [closo-B₁₂H₁₂]²⁻and [closo-1-CB₁₁H₁₂]⁻, and the three isomeric diboranes, such as[closo-B₁₂Cl₁₂]²⁻ (Knoth, W. H., Miller, H. C., Sauer, J. C., Balthis,J. H., Chia, Y. T., Muetterties, E. L., Inorg, Chem, 1964, 3, 159-167),[closo-CB₁₁(CH₃)₁₂]⁻, (King, B. T.; Janousek, Z.; Grüner, B.; Trammell,M.; Noll, B. C.; Michl, J. J. Am. Chem. Soc. 1996, 118, 10902-10903) andcloso-1,12-C₂B₁₀(CH₃)₁₂, (W. Jiang, C. B. Knobler, M. D. Mortimer, M. F.Hawthorne, Angew. Chem. 1995, 107, 1470-1473; Angew. Chem. Int. Ed.Engl. 1995, 34, 1332-1334.) have been synthesized. The existence andformulation of similar highly substituted polyhedral borane derivativeshaving hydrophilic substituents, such as hydroxyl have most recentlybeen demonstrated by Hawthorne and Peyman (“Aromatic PolyhedraHydroxyborates: Bridging Boron Oxides and Boron Nitrides”, Angew. Chem.Int. Ed., 1999,38,1061-1064).

U.S. Pat. No. 3,551,120 to Miller, et.al. discloses the formation ofnumerous ionic icosahedral substituted boranes of the general formulaM_(a)(B₁₂H_(12-y)X_(y))_(b), with y=1-12, and U.S. Pat. No. 3,390,966 toKnofl et. al. discloses the formation of numerous ionic carboranes ofthe general formula M_(a)(B₁₀H_(10-y)X_(y))_(b), with y=1-10, where b=1to 3. However, examples of the analogous closo-borane dianions[closo-B_(n)(CH₃)_(n)]²⁻ (n=6-12) have not been reported.

Paramagnetic persubstituted polyhedral closo-boranes such as[closo-B₆X₆]⁻ (Heinrich, A, Keller, H. L., Preetz, W. Z. Naturforsch.,Teil B., 1990, 45, 184) and [closo-B₉X₉]⁻ (Wong, E. H., Kabbani, R. M.,Inorg. Chem., 1990, 45, 184) where X is Cl, Br or I and[closo-CN₁₁Me₁₂]⁻ (B. T. King, B. T., Noll, B. C., McKinley, A. J.,Michl, J., J. Am. Chem. Soc., 1996. 118. 10902) have also been reported.These species were obtained via metal-ion oxidation of the correspondingreduced borane clusters. In each case, solutions of these paramagneticspecies are moderately stable, but decolorize after prolonged contactwith air.

Many researchers have sought globular structures possessing bothhydrophobic surfaces and extraordinary kinetic stability with which tosynthesize supramolecular structures, weakly-coordinating anions andspace-controlling drug components. The fullerenes, characterized byunique chemistry and physical properties, represent one family of suchprecursors. Another family of globular hydrophobes, referred to as“camouflaged” carboranes, have been described in the literature (Jiang,W.; Knobler, C. B.; Hawthorne, M. F. Angew. Chem., Int. Ed. Engl. 1995,34, 1332-1334; King, B. T.; Janousek, Z.; Grüner, B.; Trammell, M.;Noll, B. C.; Michl, J. J. Am. Chem. Soc. 1996, 118, 3313-3314; Herzog,A.; Maderna, A.; Knobler, C. B.; Hawthorne, M. F. Chem. Eur. J. 1999,5,1212-1217). These species may approach the van der Waals diameter ofC₆₀ by attachment of methyl groups and functionalized methylsubstituents to the icosahedral scaffolding of the aromatic[closo-C_(n)B_(12−n)H₁₂]^(n−2) (n=1 or 2). Whereas hydrophobic andamphiphilic derivatives of this sort are known with n=1 or 2, the fullymethylated derivative of the parent species,dodecamethyl-closo-dodecaborate(2−), (n=0), has not been shown.

BEIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the molecular structure of thedodecamethyl-closo-dodecaborate(2−) anion [closo-B₁₂(CH₃)₁₂]²⁻.

FIG. 2 shows the molecular structure of thedodecamethyl-closo-dodecaborate(1−) anion [closo-B₁₂(CH₃)₁₂]⁻.

DETAILED DESCRIPTION

The closo-borane dianions [closo-B_(n)(CH₃)_(n)]²⁻ (n=6-12) have nowbeen made using a new permethylation technique. Icosahedralcloso-boranes are treated with trimethylaluminum and methyl iodide inthe absence of a solvent. This method produced the permethyldodecaborateion, [closo-B₁₂(CH₃)₁₂]²⁻, dodecamethyl-closo-dodecaborate(2−) and theparamagnetic compound, [closo-B₁₂(CH₃)₁₂]⁻ when applied to the parentanion of the polyhedral borane family, aromatic [closo-B₁₂H₁₂]²⁻. Theanion and dianion were characterized by ¹H and ¹¹B NMR spectroscopy,high-resolution fast atom bombardment (FAB) mass spectrometry, cyclicvoltammetry, and single-crystal X-ray diffraction.

The prior art describes the reaction of methyl trifluoromethanesulfonateor aluminum chloride/methyl iodide with carboranes (Jiang, W.; Knobler,C. B.; Hawthorne, M. F. Angew. Chem., Int. Ed. Engl., 1995, 34,1332-1334; King, B. T.; Janousek, Z.; Grüner, B.; Trammell, M.; Noll, B.C.; Michl, J. Am. Chem. Soc. 1996, 118, 3313-3314.; Herzog, A.; Maderna,A.; Knobler, C. B.; Hawthorne, M. F. Chem. Eur. J, 1999, 5,1212-1217).However, when methyl trifluoromethanesulfonate was added to[N(n-Bu)₄]₂[closo-B₁₂H₁₂]²⁻ only partial triflation was obtained andonly partial halogenation resulted when aluminum chloride/methyl iodidewas used. It has now been found that a mixture of [closo-B₁₂H₁₂]²⁻,trimethylaluminum, and methyl iodide held at 45° C. for about 20 to 150hours produced various polyiodinated species [closo-B₁₂Me_(12-x)I_(x)]²⁻(5≦×≦12). Heating of the suspension at 45° C. for about 6 days(140-150hours) gave a mixture of [closo-B₁₂(CH₃)₁₂]²⁻, and[closo-B₁₂(CH₃)₁₁I]²⁻. This mixture was then isolated and again heatedat the reflux temperature in neat trimethylaluminum to convert theby-product [closo-B₁₂(CH₃)_(12-x)I_(x)]²⁻ into [closo-B₁₂(CH₃)₁₂]²⁻. Theblue colored, air stable paramagnetic ion closo-B₁₂(CH₃)₁₂ ⁻ is theneasily obtained from the dianion by chemical oxidation usingceric(4)-ammonium nitrate in acetonitrile (CAN).

Following CAN oxidation of [NBu^(n) ₄]₂[closo-B₁₂(CH₃)₁₂](TBA)₂[closo-B₁₂(CH₃)₁₂] in acetonitrile, the anion [closo-B₁₂(CH₃)₁₂]⁻was isolated as a PPN salt in 66% yield. Reduction of[closo-B₁₂(CH₃)₁₂]⁻ with NaBH₄ in ethanol regenerated B₁₂(CH₃)₁₂]²⁻ ingood yield.

To prepare (Ph₃P═N═PPh₃)[closo-B₁₂(CH₃)₁₂] a sample of [NBu^(n) ₄]₂,also known as (TBA)₂, (0.23 g, 0.29 mmol) was dissolved in acetonitrile(5 ml) and an acetonitrile solution (5 ml) of ceric(IV) ammonium nitrate(0.16 g, 0.29 mmol) was added. The reaction mixture, which immediatelyturned deep blue, was stirred for 5 min and then added to 50 ml ofwater. The precipitate was separated by filtration, dissolved in ethanol(10 ml) and again filtered. (Ph₃P═N═PPh₃)Cl (1.70 g., 2.90 mmol)dissolved in ethanol (10 ml) was added to the filtrate. Upon coolingovernight at about 0° C. dark blue (Ph₃P═N═PPh₃)[closo-B₁₂(CH₃)₁₂] (0.16g., 0.19 mmol., 66% yield) separated by crystallization.

It appears that the iodine atoms of the B-I vertices are successivelyexchanged by methyl groups when the species [closo-B₁₂Me_(12-x)I_(x)]²⁻(x≦5) are heated in trimethylaluminum. This was verify by heating themonoiodinated anion [closo-B₁₂H₁₁I]²⁻ to reflux in neattrimethylaluminum and [closo-B₁₂H₁₁(CH₃)]²⁻ was obtained in 55% yield.This procedure provides an alternate route to monoalkylated[closo-B₁₂H₁₁R]²⁻ anions, which are usually obtained throughpalladium-catalyzed alkylation of [closo-B₁₂H₁₁I]²⁻ with Grignardreagents (Peymann, T.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem.1998, 37, 1544-1548). No reaction was observed when[PPh₄]₂[closo-B₁₂I₁₂] was heated for 4 days in neat trimethylaluminum atthe reflux temperature.

The NMR data for [closo-B₁₂(CH₃)₁₂]²⁻ (¹¹B NMR: singlet at −10.8 ppm, ¹HNMR: broad singlet at −0.48 ppm) is in accordance with its symmetry(point group I_(h)). Because of quadrapole moment of the boron nucleus,a ¹³C NMR resonance for the B-CH₃carbon atoms of [closo-B₁₂(CH₃)₁₂]²⁻was not obtained. High-resolution FAB mass spectrometry confirmed themass of the permethylated derivative [closo-B₁₂(CH₃)₁₂] (centered atm/z=310.4020 with the correct isotopic distribution). The cyclicvoltammogram of [Et₄N]₂[closo-B₁₂(CH₃)₁₂] (100 mM Et₄NPF₆, Ag/AgCl,acetonitrile) shows a reversible one-electron oxidation process[closo-B₁₂(CH₃)₁₂]²⁻/[closo-B₁₂]⁻ at E_(½)=0.41 V. The dinegativespecies [closo-B₁₂(CH₃)₁₂]²⁻ is more easily oxidized than the monoanion[closo-CB₁₁(CH₃)₁₂]⁻ (E_(pa)=1.6 V) (King, B. T.; Janousek, Z.; Grüner,B.; Trammell, M.; Noll, B. C.; Michl, J. J. Am. Chem. Soc. 1996, 118,3313-3314).

The dodecamethyl closo-borane anion [closo-B₁₂(CH₃)₁₂]⁻ can be producedby

a) heating a solution of [closo-B₁₂H₁₂]²⁻, trimethylaluminum, and methyliodide at 45° C. for about 140 to about 150 hours to produce a mixtureof [closo-B₁₂(CH₃)₁₂]²⁻ and [closo-B₁₂(CH₃)₁₁I]²⁻

b) heating the mixture at reflux temperature in neat trimethylaluminumto produce [closo-B₁₂(CH₃)₁₂]²⁻, and

c) reacting [closo-B₁₂(CH₃)₁₂]²⁻ with ceric(4) ammonium nitrate inacetonitrile to produce [closo-B₁₂(CH₃)₁₂]⁻.

By starting with [closo-B_(n)H_(n)]⁻, where n=6-12, various differentcompounds of the formula [closo-B_(n)(CH₃)_(n)]⁻ can be produced.

The anion [closo-B₁₂(CH₃)₁₂]⁻ was characterized by high resolution fastatom bombardment mass spectrometry (HR-FAB-MS), electron paramagneticresonance (EPR). UV-VIS spectroscopy and cyclic voltammetry.Furthermore, the crystal structure of [Ph₃P═N═PPh₃closo-B₁₂(CH₃)₁₂] wasdetermined by single crystal X-ray diffraction. Negative HR-FAB-MS:found: mlz=310.4022: calc. 310.4014. Crystal data for(Ph₃P═N═PPh₃)[closo-B₁₂(CH₃)₁₂] is: C₄₈H₆₆B₁₂NP₂, M=878.04, monoclinic.a=3466(3), b=934.2(8), c=1869(2) pm. U=5.120(7)nm³, T=298°K, space groupC2/cZ=4 μ(Mo-Kα)=1.2 cm⁻¹. 4900 unique reflections were measured, 2489reflections were considered observed [I>2α(I)] and all data was used inall calculations. The final R factor (R=Σ| |F_(o)|−|F_(c)||/Σ|F_(o).)was 0.066 (for 2489 independent reflections). The structure was solvedusing statistical methods and refined by full-matrix least squares onF².CCDC 182/1405. Crystallographic files are available in cif format athttp://www.rsc.org/suppdata/cc/1999/2039/

Blood-red single-crystals of [(C₅H₅N)₂CH₂]closo-B₁₂(CH₃)₁₂] CH₃CN.(orthorhombic Pc2₁n, a=971.5(7) pm, b=1505.2(10) pm, c=2281.9(15) pm;Z=4; R=0.074, R_(w),=0.184; GOF=1.04) were obtained fromacetonitrile/ethanol. The crystal structure of [closo-B₁₂(CH₃)₁₂]²⁻,shown in FIG. 1, confirms the permethylation of the B12 icosahedron withsome distortion of its icosahedral geometry. The thermal ellipsoids inFIG. 1 represent a 30% probability level. The selected bond distances(pm) of the anion are: B-B=174.0(14)-181.1(14); B-C=159.1(14)-170.4(13).The B-B bond lengths of [closo-B₁₂(CH₃)₁₂]²⁻ are similar to the bonddistances of the unsubstituted anion [closo-B₁₂H₁₂]²⁻, reported byWunderlich and Lipscomb (175.5(7)-178.0(7) pm (Wunderlich, J. A.;Lipscomb, W. N. J. Am. Chem. Soc. 1960, 82, 4427-4428). The B-C bonddistances of [closo-B₁₂(CH₃)₁₂]²⁻ are longer than the B-C bond of[closo-B₁₂H₁₁(CH₃]²⁻, (158(2) pm; Peymann, T.; Knobler, C. B.;Hawthorne, M. F. Inorg. Chem. 1998, 37, 1544-1548), the exo B-C bonds of[closo-CB₁₁(CH₃)₁₂]⁻ (159(2)160.1(6) pm (King, B. T.; Janousek, Z.;Grüner, B.; Trammell, M.; Noll, B. C.; Michl, J., J. Am. Chem. Soc.1996, 118, 3313-3314) and closo-1,12-C₂B₁₀(CH₃)₁₂ (158.3(6) pm (Jiang,W.; Knobler, C. B.; Hawthorne, M. F. Angew. Chem., Int. Ed Engl. 1995,34, 1332-1334). The red color of [(C₅H₅N)₂CH₂] [closo-B₁₂(CH₃)₁₂] isapparently due to a charge-transfer interaction of the anion[closo-B₁₂(CH₃)₁₂]²⁻ with the pyridinium rings of the dipositive cation.The plane through the triangle B1, B4, and B5 is nearly parallel to theplane established by a pyridinium ring N1 and C2 through C6. The anglebetween the normals of these two planes is 7.3°. The distances of theboron atoms B1, B4, and B5 from the latter plane are 508(1), 486(1), and504(1) pm, respectively and the distances of the methyl carbon atomsC1M, C4M, and C5M are 389(1), 338(1), and 378(1) pm, respectively.

The longest across-cage methyl carbon-methyl carbon distances of[closo-B₁₂(CH₃)₁₂]²⁻ average 668 pm; the corresponding maximum methylhydrogen-methyl hydrogen distance is 761 pm compared to 707 pm for C₆₀(Liu, S.; Lu, Y.; Kappes, M. M.; Ibers, J. A. Science 1991, 254,408-410).

The radicals differ significantly in reactivity as their persistencelargely depends upon the unpaired electron's chemical and physicalenvironment. (Griller, D., Ingold, K. U., Acc.,Chem.Res, 1976, 9.13). Amajor effect that stabilizes paramagnetic species is steric crowding. Aradical center surrounded by bulky groups is more persistent thansimilar species without this protection. This deep blue radical-anion issurprisingly stable with respect to reaction with oxygen.

A solid sample of (Ph₃P═N═PPh₃) [closo-B₁₂(CH₃)₁₂] exhibits an EPRsignal with g=2.0076. The UV-VIS spectrum (FIG. 1) of the blue salt[NEt₄] [closo-B₁₂(CH₃)₁₂] (TEA2) in acetonitrile displays absorption inthe visible region.

Cyclic voltammetry (100 mM NEt₄PfF₆, Ag/AgCl, MeCN) of[closo-B₁₂(CH₃)₁₂]³¹ reveals a reversible wave with E½=0.44 V for theone-electron process 2+e⁻→1. The reduction potential of[closo-B₁₂(CH₃)₁₂]⁻ matches the corresponding oxidation potentialpreviously determined for [closo-B₁₂(CH₃)₁₂]²⁻. The X-ray crystalstructure (FIG. 2) confirms that [closo-B₁₂(CH₃)₁₂]⁻ is a permethylatedmonoanionic closo-borane.

The crystal structure of the anion is shown in FIG. 2. in the solidstate, the anionic cluster of (Ph₃P═N═PPh₃) [closo-B₁₂(CH₃)₁₂]⁻ is lessdistorted from icosahedral symmetry than the dianionic species[closo-B₁₂(CH₃)₁₂]⁻ studied as a [(Ph₃P═N═PPh₃)C₅H₄N)₂ CH²]²⁺ salt [B-Bbond lengths: [closo-B₁₂(CH₃)₁₂]⁻=178.5(8)-180.5(7)pm;[closo-B₁₂(CH₃)₁₂]²⁻=174(1-B-C bond lengths:[closo-B₁₂(CH₃)₁₂]⁻=159.8(9)-161.3(8)pm;1[closo-B₁₂(CH₃)₁₂]²⁻=159(1)-170(1) pm]. The maximum across-cage methylcarbon separations average 668 pm for the dianionic species compared to663 pm for the monoanion. The greater distortion of [closo-B₁₂(CH₃)₁₂]²⁻may be explained by the charge-transfer interaction of the dipositive[[closo-B₁₂(CH₃)₁₂] (C₅H₄N)₂CH₂]²⁺ counter ion and the dianion. Thisinteraction is suggested by the unique color of the blood-red[closo-B₁₂(CH₃)₁₂(C₅H₄N)CH₂] salt.

Because the new dodecaborate moieties, [closo-B₁₂(CH₃)₁₂]²⁻ and theparamagnetic ion [closo-B₁₂(CH₃)₁₂]⁻, are weakly coordinated anions withlipophilic properties they are suitable for extraction of radioactivemetal ions from nuclear waste. For example, the [closo-B₁₂(CH₃)₁₂]²⁻ ionforms hydrocarbon soluble salts with cations such as cesium andstrontium. [closo-B₁₂(CH₃)₁₂]²⁻ dissolved in kerosene or otherhydrocarbon solvents can be used to extract ¹³⁷Cs⁺ and ⁹⁰Sr²⁺ fromaqueous radioactive waste. A representative extraction formula, where Mis the extracted metal, is as follows:

M²⁺+(H₃O⁺)₂B₁₂(CH₃)₁₂ ²⁻→M²⁺B₁₂(CH₃)₁₂ ²⁻+2H₃O⁺→M²⁺+(H₃O⁺⁾ ₂B₁₂(CH₃)₁₂²⁻

The crude waste acid water phase is mixed with permethyldodecaboratesalt. The hydrocarbon phase will then contain the complexed metal. Waterwas then mixed with the hydrocarbon phase to strip out the M²⁺ andregenerate the H₃O⁺ salt dissolved in water.

Also the permethyldodecaborates can serve as precursors for newpharmacophores, resulting in new drugs. Still further, they can be usedto produce stabilized unilamellar boron-containing liposomes fordelivery to tumors for boron neutron capture therapy. The paramagneticlipophilic anion, because of its blue color and response to a magneticor electric field, is further useful in sensor systems to detect easilyoxidized or reduced compounds and, because it can function as anreversible electron trap (oxidant) could be incorporated inelectrochemical systems.

[closo-B₁₂(CH₃)₁₂]⁻ presents unique possibilities inherent in theconcept of camouflaged polyhedral boranes. Here the persubstitutionstabilizes an unusual oxidation state that has not been obtained fromthe ‘naked’ parent species [closo-B₁₂H₁₂]²⁻. Upon electrochemicaloxidation (E_(½)1.43 V vs SCE), the dianion instead dimerizes by anundetermined mechanism with loss of one exo H-atom and dimerization ofB₁₂-cages to form the B-H-B bridge of [B₂₄H₂₃]³⁻ (Wiersema, R. J.,Middaugh, R. L., Inorg. Chem., 1969, 8. 2074):

2[closo-B₁₂H₁₂]²⁻→[B₂₄H₂₃]³⁻+H⁺+2e⁻

The latter species is structurally related to [a²-B₂₀H₁₉]³⁻.(Watson-Clark, R. A., Knobler, C. B. and Hawthorne, M. F., Inorg. Chem.,1996, 35, 2963). This reaction pathway is not available for thepermethylated radical.

A further advantage of the disclosed anions and there preparation arethat they are easy to synthesize and therefore much less costly thenprior available compounds such as produced by Michl. The anionsdisclosed herein are readily derived from (B₁₂H₁₂)²⁻. The Michl speciesis expensive to make because it is derived from closo-(CB₁₁H₁₂)⁻ whichis difficult, and therefore expensive, to produce. Additionally, thewater solubility of the resultant compounds provides a novel startingmolecule for drug design. Other B_(n)H_(n) ²⁻ ions, for example B₁₀H₁₀²⁻, might be permethylated using the above described procedure.Metallocarborane and metalloborane anions are expected to undergo thesame reactions and are possible substrates for methylation using thisprocedure, It is further contemplated that compounds such as N-bromo- orN-chlorosuccinimide could be used in the described procedure to producea halomethyl derivative, such as [B₁₂(CH₃)₁₁CH₂Cl]²⁻.

We claim:
 1. Dodecamethyl closo-borane dianion [closo-B₁₂(CH₃)₁₂]²⁻. 2.Dodecamethyl closo-borane anion, [closo-B₁₂(CH₃)₁₂]⁻.
 3. A method ofproducing the dodecamethyl closo-borane dianion [closo-B₁₂(CH₃)₁₂]²⁻comprising a) heating a solution of [closo-B₁₂H₁₂]²⁻, trimethylaluminum,and methyl iodide at 45° C. for about 140 to about 150 hours to producea mixture of [closo-B₁₂(CH₃)₁₂]²⁻ and [closo-B₁₂(CH₃)₁₁]²⁻, and b)heating the mixture at reflux temperature in neat trimethylaluminum toproduce [closo-B₁₂(CH₃)₁₂]²⁻.
 4. A method of producing the dodecamethylcloso-borane anion [closo-B₁₂(CH₃)₁₂]⁻ comprising: a) heating a solutionof [closo-B₁₂H₁₂]²⁻, triethylaluminun, and methyl iodide at 45° C. forabout 140 to about 150 hours to produce a mixture of[closo-B₁₂(CH₃)₁₂]²⁻ and [(closo-B₁₂(CH₃)₁₁I]²⁻, b) heating the mixtureat reflux temperature in neat trimethylaluminum to produce[closo-B₁₂(CH₃)₁₂]²⁻, and c) reacting [closo-B₁₂(CH₃)₁₂]²⁻ with ceric(4)ammonium nitrate in acetonitrile to produce [(closo-B₁₂(CH₃)₁₂]⁻.
 5. Themethod of producing the methylated closo-borane dianions[closo-B₁₂Me_(12-x)I_(x)]²⁻ where x is from 5 to 12 comprising, heatinga solution of [closo-B₁₂H₁₂]²⁻, trimethylaluminum, and methyl iodide at45° C. for a period of time greater than about 20 but less than about150 hours.
 6. A method of producing a methyl closo-borane dianion[closo-B_(n)(CH₃)_(n)]²⁻where n=6-12, comprising: a) heating a solutionof [closo-B_(n)H_(n)]²⁻, trimethylaluminum, and methyl iodide at 45° C.for about 140 to about 150 hours to produce a mixture of[closo-B_(n)(CH₃)_(n)]²⁻ and [closo-B_(n)(CH₃)_(n−1)I]²⁻, and b) heatingthe mixture at reflux temperature in neat trimethylaluminum to produce[closo-B_(n)(CH₃)_(n)]²⁻.
 7. A method of producing a methyl closo-boraneanion [closo-B_(n)(CH_(n))₁₂]⁻ where n=6-12, comprising: a) heating asolution of [closo-B_(n)H_(n)]²⁻, trimethylaluminum, and methyl iodideat 45° C. for about 140 to about 150 hours to produce a mixture of[closo-B_(n)(CH₃)_(n)]²⁻ and [closo-B_(n)(CH₃)_(n−1)I]²⁻, b) heating themixture at reflux temperature in neat trimethylaluminum to produce[closo-B_(n)(CH₃)_(n)]²⁻, and c) reacting [closo-B_(n)(CH₃)_(n)]²⁻ withceric(4) ammonium nitrate in acetonitrile to produce[closo-B_(n)(CH₃)_(n)]⁻.